1. Field of the Invention
The present invention relates to a video signal converting apparatus for decoding a high definition television signal being transmitted in band-width compression by employing the sub-Nyquist sampling method, and for converting it into a conventional standard television signal.
2. Description of the Prior Art
A high definition television signal has a band-width of 20 MHz or more, so that it is necessary to compress the band when transmitting such a signal through a direct broadcasting satellite or the like. As the effective technology for greatly compressing the band-width of a high definition television signal, the method of compressing the signal band by sub-Nyquist sampling is known, and the MUSE (Multiple Sub-Nyquist sampling encoding) method, and the HD-MAC (High Definition - Multiple Analogue Component) method have been proposed.
(See, for example, Nrnomiya et al., "Satellite one-channel transmission method of high definition television (MUSE)," Television Academic Society Technical Report TEBS95-2, pages 37 to 42, F.W.P. Vreeswijk et al., "HD-MAC Coding for Compatible Broadcasting of HDTV Signals," Symposium record Broadcast Session 1989, pp. 37-53.)
These methods are, as shown in FIG. 9, perform offset sub-sampling between fields and between frames, and of the sampling points of the original signal, 1/4 of the sampling points are transmitted, and a complete picture is transmitted in four fields, and the band-width of the original signal is compressed to 1/4. At the receiving side, from the transmitted sampling points, non-transmitted sampling points (non-sampling points) are obtained by interpolation, and the original signal is restored. For example, in the stationary area, all sampling points of the present field and the past three fields are used in interpolation. In the moving area, only the sampling points in the transmitted field are interpolated.
Thus, the method of interpolation differs between the stationary area and moving area, and the interpolation method is changed over by detecting the motion of the picture.
By such processing at the transmission side and reception side, high definition television broadcast can be enjoyed. However, the signal processing circuit (i.e., decoder) at the receiving side is very expensive, and it will take a very long time until it is widely adopted for household use. It is accordingly considered to use the television and VCR of the conventional standard method presently used widely at home, but in such a case, it is impossible to display and record in the high definition television signal state, and it is necessary to convert the high definition television signal into the conventional television signal by using a standard converter.
Hitherto, a video signal converter has been disclosed, for example, in "MUSE/NTSC Converter," IEEE Trans. CE-35,3, pp. 142 to 152.
In such an existing video signal converter, when interpolating in order to restore the band-compressed signal by offset sub-sampling, only the intra-field interpolation corresponding to the moving area processing by the decoder is performed from the viewpoint of the circuit scale and the cost, the signal transmission characteristics will be as shown in the frequency characteristics in FIG. 10 (a), and in the stationary area, the high resolution components of the high definition television signal by the offset sampling of the MUSE system as shown in the frequency characteristics in FIG. 10 (b) are left over as the aliasing components in the converted conventional standard television signal, which results in deterioration of the picture quality.